U.S. patent number 6,395,197 [Application Number 09/732,451] was granted by the patent office on 2002-05-28 for hydrogen and elemental carbon production from natural gas and other hydrocarbons.
This patent grant is currently assigned to Bechtel BWXT Idaho LLC. Invention is credited to Brent A. Detering, Peter C. Kong.
United States Patent |
6,395,197 |
Detering , et al. |
May 28, 2002 |
Hydrogen and elemental carbon production from natural gas and other
hydrocarbons
Abstract
Diatomic hydrogen and unsaturated hydrocarbons are produced as
reactor gases in a fast quench reactor. During the fast quench, the
unsaturated hydrocarbons are further decomposed by reheating the
reactor gases. More diatomic hydrogen is produced, along with
elemental carbon. Other gas may be added at different stages in the
process to form a desired end product and prevent back reactions.
The product is a substantially clean-burning hydrogen fuel that
leaves no greenhouse gas emissions, and elemental carbon that may
be used in powder form as a commodity for several processes.
Inventors: |
Detering; Brent A. (Idaho
Falls, ID), Kong; Peter C. (Idaho Falls, ID) |
Assignee: |
Bechtel BWXT Idaho LLC (Idaho
Falls, ID)
|
Family
ID: |
22629971 |
Appl.
No.: |
09/732,451 |
Filed: |
December 6, 2000 |
Current U.S.
Class: |
252/373;
423/418.2; 423/648.1; 423/650; 423/445R |
Current CPC
Class: |
B01J
19/088 (20130101); C01B 32/05 (20170801); C01B
3/34 (20130101); H01M 8/0618 (20130101); B01J
19/26 (20130101); C01B 3/24 (20130101); H01M
8/0631 (20130101); B01J 2219/00103 (20130101); B01J
2219/00159 (20130101); B01J 2219/0875 (20130101); B01J
2219/0898 (20130101); B01J 2219/00132 (20130101); B01J
2219/0894 (20130101); B01J 2219/0809 (20130101); Y02P
20/129 (20151101); B01J 2219/00108 (20130101); B01J
2219/00119 (20130101); B01J 2219/00094 (20130101); B01J
2219/0845 (20130101); B01J 2219/083 (20130101); Y02E
60/50 (20130101); B01J 2219/0879 (20130101); B01J
2219/0826 (20130101); B01J 2219/00166 (20130101) |
Current International
Class: |
C01B
3/24 (20060101); C01B 3/00 (20060101); B01J
19/08 (20060101); B01J 19/26 (20060101); C01B
31/00 (20060101); C01B 31/02 (20060101); C01B
3/34 (20060101); C07C 001/02 (); C01B 031/18 ();
C01B 031/00 (); C01B 003/02 (); C01B 003/24 () |
Field of
Search: |
;252/373
;423/418.2,445R,648.1,650 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0 618 951 |
|
Sep 1996 |
|
EP |
|
PCT/NO92/00200 |
|
Jan 1992 |
|
WO |
|
Primary Examiner: Richter; Johann
Assistant Examiner: Parsa; J.
Attorney, Agent or Firm: Workman Nydegger & Seeley
Government Interests
CONTRACTUAL ORIGIN OF THE INVENTION
This invention was made with United States Government support under
Contract No. DE-AC07-94ID13223, now Contract No. DE-AC07-99ID13727
awarded by the United States Department of Energy. The United
States Government has certain rights in the invention.
Parent Case Text
RELATED APPLICATION
This application claims priority from United States provisional
application Ser. No. 60/172,976 filed on Dec. 21, 1999 and is
incorporated by reference.
Claims
We claim:
1. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles, comprising:
introducing a hydrocarbon reactant stream at one end of an axial
reactor;
heating the hydrocarbon reactant stream as the hydrocarbon reactant
stream flows axially toward an outlet end of the axial reactor,
wherein the axial reactor has a length and a temperature and is
operated under conditions sufficient to effect heating of the
hydrocarbon reactant stream to a selected reaction temperature at
which at least diatomic hydrogen is available as an intermediate
product stream at a location adjacent the outlet end of the axial
reactor;
directing the intermediate product stream through a convergent
nozzle arranged at the outlet end of the axial reactor; and
directing the intermediate product stream through a means for rapid
gas expansion, thereby creating a recirculation of the intermediate
product stream which results in a local reheating of the
intermediate product stream as the intermediate product stream
flows axially through the means for rapid gas expansion, wherein
the reheating is sufficient to decompose a hydrocarbon in the
intermediate product stream to an end product comprising at least
one of diatomic hydrogen and elemental carbon.
2. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, further comprising:
cooling the at least one end product and any remaining intermediate
product stream exiting from the means for rapid gas expansion.
3. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, wherein the hydrocarbon
reactant stream is selected from methane, natural gas, and light
hydrocarbons.
4. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, wherein first heating the
hydrocarbon reactant stream comprises:
forming a plasma gas for heat exchange with the hydrocarbon
reactant stream, wherein the plasma gas is selected from argon,
hydrogen, methane, natural gas, hydrocarbons, and combinations
thereof.
5. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, wherein first heating the
hydrocarbon reactant stream further comprises:
heating the hydrocarbon reactant stream to about 2,000.degree.
C.
6. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, wherein first heating the
hydrocarbon reactant stream further comprises:
configuring the length of the axial reactor to substantially
prevent the formation within the axial reactor of unsaturated
hydrocarbons with more than two carbons therein.
7. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, wherein expanding the
intermediate product stream comprises:
expanding the intermediate product stream through a
convergent-divergent nozzle.
8. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, wherein expanding the
intermediate product stream comprises:
expanding the intermediate product stream through a
convergent-divergent nozzle, wherein the divergent nozzle has an
angle of about 35.degree. or greater.
9. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, wherein expanding the
intermediate product stream comprises:
expanding the intermediate product stream through a convergent
nozzle and into a free-expansion chamber.
10. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, wherein expanding the
intermediate product stream comprises:
supplying the intermediate product stream through the convergent
nozzle; and
blending the intermediate product stream with a mixing gas to cause
individual species from the intermediate product stream to become
separated by the mixing gas.
11. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, wherein expanding the
intermediate product stream comprises:
supplying the intermediate product stream through the nozzle at a
velocity at which kinetic temperature drops by an absolute
temperature factor of greater than about two.
12. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, wherein expanding the
intermediate product stream comprises:
expanding the intermediate product stream through the nozzle at a
velocity at which kinetic temperature drops by an absolute
temperature factor of greater than about three.
13. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, wherein expanding the
intermediate product stream comprises:
expanding the intermediate product stream through the nozzle at a
velocity at which kinetic temperature drops by an absolute
temperature factor of greater than about four.
14. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, wherein second heating the
intermediate product stream further comprises:
heating the expanded intermediate product stream by an absolute
temperature factor of greater than about 1.25.
15. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, wherein second heating the
intermediate product stream further comprises:
heating the expanded intermediate product stream by an absolute
temperature factor of about 1.4.
16. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, wherein cooling the at least
one end product and remaining intermediate product further
comprises:
preheating the hydrocarbon reactant stream with the at least one
end product and any remaining intermediate product.
17. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, comprising:
cooling at least some of the diatomic hydrogen by heat exchange
with some of the hydrocarbon reactant stream; and
supplying the diatomic hydrogen to a fuel cell.
18. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, further comprising:
cooling at least some of the diatomic hydrogen by heat exchange
with some of the hydrocarbon reactant stream;
supplying the diatomic hydrogen to a fuel cell; and
preheating at least some of the hydrocarbon reactant stream by heat
exchange with waste heat effluent from the fuel cell.
19. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, comprising:
cooling at least some of the diatomic hydrogen by heat exchange
with some of the hydrocarbon reactant stream; and
supplying the diatomic hydrogen to an internal combustion
engine.
20. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, further comprising:
cooling at least some of the diatomic hydrogen by heat exchange
with some of the hydrocarbon reactant stream;
supplying the diatomic hydrogen to an internal combustion engine;
and
preheating at least some of the hydrocarbon reactant stream by heat
exchange with waste heat effluent from the internal combustion
engine.
21. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, further comprising:
contacting the elemental carbon with a carbon dioxide source;
and
converting the elemental carbon and the carbon dioxide to carbon
monoxide.
22. A method of converting one or more hydrocarbon reactant to an
end a product in the form of a diatomic hydrogen gas or ultrafine
solid particles according to claim 1, wherein the elemental carbon
end product is in the form of ultrafine particles in a size range
from about 10 nm to about 100 nm.
23. A method for thermal conversion of carbon dioxide and methane
in a thermodynamically stable high temperature gaseous stream to
carbon monoxide and diatomic hydrogen, comprising the following
steps:
introducing a stream of plasma arc gas between the electrodes of a
plasma torch including at least one pair of electrodes positioned
at the inlet end of an axial reactor chamber, the stream of plasma
arc gas being introduced at a selected plasma gas flow while the
electrodes are subjected to a selected plasma input power level to
produce a plasma within the reactor chamber and extending toward
its outlet end;
thoroughly mixing an incoming reactant stream into the plasma by
injecting at least methane into the reactor chamber at or adjacent
to its inlet end at a selected injection angle and at a selected
reactant input rate to progressively effect heat transfer between
the plasma and the resulting gaseous stream as it flows axially
toward the outlet end of the reactor chamber;
the length of the reactor chamber being sufficient to effect
heating of the gaseous stream to a selected equilibrium temperature
at which an intermediate product stream comprising unsaturated
hydrocarbons is available as a thermodynamically unstable reaction
product within the gaseous stream at a location adjacent to the
outlet end of the reactor chamber;
directing the intermediate product stream through a converging
section of a coaxial convergent-divergent nozzle positioned in the
outlet end of the reactor chamber to rapidly cool the gaseous
stream by converting thermal energy to kinetic energy as a result
of adiabatic and isentropic expansion as it flows axially through
the nozzle, the nozzle having a converging section and a diverging
section respectively leading to and from a restrictive open
throat;
directing the intermediate product stream through the diverging
section of the nozzle, thereby creating a recirculation of the
intermediate product stream which results in a local reheating of
the intermediate product stream as the intermediate product stream
flows axially through the diverging section of the nozzle, wherein
the reheating is sufficient to decompose the unsaturated
hydrocarbons in the intermediate product stream to an end product
comprising at least one of diatomic hydrogen and elemental carbon;
and
cooling the gaseous stream exiting the nozzle by reducing its
velocity while removing heat energy at a rate sufficient to prevent
increases in its kinetic temperature.
24. A method for thermal conversion of carbon dioxide and methane
in a thermodynamically stable high temperature gaseous stream to
carbon monoxide and diatomic hydrogen according to claim 23,
comprising:
contacting the elemental carbon with a carbon dioxide source;
and
converting the elemental carbon and the carbon dioxide to carbon
monoxide.
25. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles, comprising:
introducing a hydrocarbon reactant stream at one end of an axial
reactor;
first heating the hydrocarbon reactant stream as the hydrocarbon
reactant stream flows axially toward an outlet end of the axial
reactor, wherein the axial reactor has a length and a temperature
and is operated under conditions sufficient to effect heating of
the hydrocarbon reactant stream to a selected reaction temperature
at which at least diatomic hydrogen is available as an intermediate
product stream at a location adjacent the outlet end of the axial
reactor;
expanding the intermediate product stream by directing the
intermediate product stream first through a convergent nozzle
arranged at the outlet end of the axial reactor and second through
a means for rapid gas expansion, thereby retaining the desired
intermediate product stream;
second heating the intermediate product stream as the intermediate
product stream flows axially through the means for rapid gas
expansion sufficient to decompose a hydrocarbon in the intermediate
product stream to a diatomic hydrogen and elemental carbon end
product;
cooling at least some of the diatomic hydrogen by heat exchange
with some of the hydrocarbon reactant stream;
supplying the diatomic hydrogen to a fuel cell; and
preheating at least some of the hydrocarbon reactant stream by heat
exchange with waste heat effluent from the fuel cell.
26. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles, comprising:
introducing a hydrocarbon reactant stream at one end of an axial
reactor;
first heating the hydrocarbon reactant stream as the hydrocarbon
reactant stream flows axially toward an outlet end of the axial
reactor, wherein the axial reactor has a length and a temperature
and is operated under conditions sufficient to effect heating of
the hydrocarbon reactant stream to a selected reaction temperature
at which at least diatomic hydrogen is available as an intermediate
product stream at a location adjacent the outlet end of the axial
reactor;
expanding the intermediate product stream by directing the
intermediate product stream first through a convergent nozzle
arranged at the outlet end of the axial reactor and second through
a means for rapid gas expansion, thereby retaining the desired
intermediate product stream;
second heating the intermediate product stream as the intermediate
product stream flows axially through the means for rapid gas
expansion sufficient to decompose a hydrocarbon in the intermediate
product stream to a diatomic hydrogen and elemental carbon end
product;
cooling at least some of the diatomic hydrogen by heat exchange
with some of the hydrocarbon reactant stream; and
supplying the diatomic hydrogen to an internal combustion
engine.
27. A method of converting one or more hydrocarbon reactants to an
end product in the form of a diatomic hydrogen gas or ultrafine
solid particles, comprising:
introducing a hydrocarbon reactant stream at one end of an axial
reactor;
first heating the hydrocarbon reactant stream as the hydrocarbon
reactant stream flows axially toward an outlet end of the axial
reactor, wherein the axial reactor has a length and a temperature
and is operated under conditions sufficient to effect heating of
the hydrocarbon reactant stream to a selected reaction temperature
at which at least diatomic hydrogen is available as an intermediate
product stream at a location adjacent the outlet end of the axial
reactor;
expanding the intermediate product stream by directing the
intermediate product stream first through a convergent nozzle
arranged at the outlet end of the axial reactor and second through
a means for rapid gas expansion, thereby retaining the desired
intermediate product stream;
second heating the intermediate product stream as the intermediate
product stream flows axially through the means for rapid gas
expansion sufficient to decompose a hydrocarbon in the intermediate
product stream to a diatomic hydrogen and elemental carbon end
product;
cooling at least some of the diatomic hydrogen by heat exchange
with some of the hydrocarbon reactant stream;
supplying the diatomic hydrogen to an internal combustion engine;
and
preheating at least some of the hydrocarbon reactant stream by heat
exchange with waste heat effluent from the internal combustion
engine.
Description
TECHNICAL FIELD
The present invention relates to equipment and methods for thermal
conversion of light hydrocarbons such as natural gas and other
reactants to desired end products, particularly diatomic hydrogen
and elemental carbon. The end products may be either a gas or
ultrafine solid particles. The present invention also relates
specifically to methods for effectively producing such end
products.
BACKGROUND OF THE INVENTION
Methane and other light hydrocarbon combustibles are often found in
remote areas. Worldwide natural gas reserves have increased on an
average of about six percent annually, while natural gas
consumption has only increased about three percent annually.
The difference between known and used reserves has increased
therefore to about 4.6 quadrillion cubic feet. Known natural gas
reserves therefore have an energy equivalent of some 770 billion
barrels of oil that is equivalent to about a 29 year worldwide
supply of oil for energy purposes. Unfortunately, however, most of
the natural gas reserves are located in remote areas. Remote
natural gas reserves usually makes the economics of extraction and
removal unfeasible.
The Fischer-Tropsch process, developed early in the 20th century in
Germany, uses fossil fuels and converts the fossil fuels to liquid
synthetic gasoline species. The Fischer-Tropsch synthesis is
strongly exothermic and often requires hydrogen in the process.
Where a Fischer-Tropsch process is being conducted at remote sites
without a proper infrastructure for readily available hydrogen, the
cost of production is significantly increased by the need to bring
the hydrogen to the remote site.
As environmental concerns increase regarding greenhouse gases that
may contribute to global warming, increased interest is directed
toward finding clean burning fuels that do not produce carbon
dioxide emissions. Hydrogen as a fuel seems to be ideal as it burns
to form only water as its combustion product.
A need has long existed for converting available carbonaceous
materials to scarce liquid hydrocarbon fuels having preferred
performance characteristics in many applications, such as internal
combustion engines and other heat engines. Prior art technology
teaches converting coal to a liquid hydrocarbon fuel by gasifying
the coal to synthesis gas, hydrogenating the resulting synthesis
gas, and recovering a liquid hydrocarbon fuel from the
hydrogenation product.
A need has long existed for converting available carbonaceous
materials to scarce liquid hydrocarbon fuels having preferred
performance characteristics in many applications, such as internal
combustion engines, jet engines and open-cycle gas turbines. Thus,
for example, U.S. Pat. No. 3,986,349 teaches a process for
converting solid coal to a liquid hydrocarbon fuel by gasifying the
coal to a synthesis gas, hydrogenating the resulting synthesis gas,
and recovering a liquid hydrocarbon fuel from the hydrogenation
product. The liquid hydrocarbon fuel is used to generate power by
relatively clean combustion in an open-cycle gas turbine.
An alternative is to produce natural gas and convert it in the
field to a more utilitarian liquid hydrocarbon fuel or liquid
chemical product for local usage or for more cost-effective
transportation to remote markets. Processes for converting light
hydrocarbon gases, such as natural gas, to heavier hydrocarbon
liquids are generally known in the prior art. Such processes
typically involve the indirect conversion of methane to synthetic
paraffinic hydrocarbon compounds, wherein methane is first
converted to a synthesis gas containing hydrogen and carbon
monoxide followed by conversion of the synthesis gas to synthetic
paraffinic hydrocarbon compounds via a Fischer-Tropsch reaction.
The unconverted synthesis gas remaining after the Fischer-Tropsch
reaction is usually catalytically reconverted to methane via a
methanation reaction and recycled to the process inlet to increase
the overall conversion efficiency of the process.
Conversion of methane to a synthesis gas is often performed by
high-temperature steam reforming, wherein methane and steam are
reacted endothermically over a catalyst contained within a
plurality of externally-heated tubes mounted in a large fired
furnace. Alternatively, methane is converted to a synthesis gas via
partial-oxidation, wherein the methane is exothermically reacted
with purified oxygen. Partial oxidation using purified oxygen
requires an oxygen separation plant having substantial compression
capacity and correspondingly having substantial power requirements.
Production of the synthesis gas via either of the above-recited
methods accounts for a major portion of the total capital cost of a
plant for converting methane to paraffinic hydrocarbons.
Autothermal reforming is a lower cost method of converting methane
to a synthesis gas. Autothermal reforming employs a combination of
partial oxidation and steam reforming. The endothermic heat
required for the steam reforming reaction is obtained from the
exothermic partial oxidation reaction. Unlike the above-recited
partial oxidation reaction, however, air is used as the source of
oxygen for the partial oxidation reaction. In addition, the
synthesis gas produced by autothermal reforming contains
substantial quantities of nitrogen from the inlet air.
Consequently, it is not possible to recycle the unconverted
components contained in the process tail gas without undesirably
accumulating an excess of nitrogen within the process. Production
of a nitrogen-diluted synthesis gas via autothermal reforming or
partial-oxidation using air followed by conversion of the synthesis
gas via a Fischer-Tropsch reaction as disclosed in U.S. Pat. Nos.
2,552,308 and 2,686,195 is, nevertheless, a useful method for
obtaining synthetic hydrocarbon liquid products from methane.
U.S. Pat. No. 4,833,170 discloses another example of autothermal
reforming, wherein a gaseous light hydrocarbon is reacted with air
in the presence of recycled carbon dioxide and steam to produce a
synthesis gas. The synthesis gas is reacted in the presence of a
hydrocarbon synthesis catalyst containing cobalt to form a residue
gas stream and a liquid stream comprising heavier hydrocarbons and
water. The heavier hydrocarbons are separated from the water and
recovered as product. The residue gas is catalytically combusted
with additional air to form carbon dioxide and nitrogen which are
separated. At least a portion of the carbon dioxide is recycled to
the autothermal reforming step.
Although prior art hydrocarbon gas conversion processes such as
disclosed in U.S. Pat. No. 4,833,170 may be relatively effective
for converting the light hydrocarbon gases to heavier hydrocarbon
liquids, such processes have not been found to be entirely cost
effective due to significant capital equipment and energy costs
attributable to compression of the inlet air. The power required to
compress the inlet air represents the majority of the mechanical
power required to operate the process, yet much of this power is
essentially lost as unrecovered pressure energy in the residue gas
from the process. The inlet air requiring compression contains
substantial quantities of nitrogen that remain essentially
chemically inert as the nitrogen passes through the process,
ultimately exiting the process in the residue gas. Furthermore,
although the residue gas has a significant chemical-energy fuel
value attributable to the carbon monoxide, hydrogen, methane and
heavier hydrocarbon components thereof, the residue gas is very
dilute, having a low heating value that renders it very difficult
and costly to recover the energy of the fuel value of the residue
gas with high efficiency. Thus, it is apparent that a need exists
for a more cost effective hydrocarbon gas conversion process.
In the above-mentioned technologies, it is well known that a
carbon-containing chemical species will be combusted and the amount
of greenhouse gases that are emitted to the atmosphere, namely
carbon dioxide, is increased. What is needed in the art is a
process for making synthetic fuels from light hydrocarbons that
eliminates the emission of greenhouse gases.
Another problem that occurs is the need for cryogenic storage of
converted fuels such as hydrogen. Hydrogen storage presents a
problem because an on-board system of a hydrogen heat engine for a
vehicle has a range of approximately 50 miles with hydrogen stored
in pressurized tanks. Typically, a combination of both cryogenic
and high pressure hydrogen storage are required in order to contain
the hydrogen in a compact enough package to carry as an on-board
system.
Several attempts have been made to store hydrogen as a metal
hydride in order to lessen the need for both cryogenic and high
pressure storage. Metal hydride storage has its own challenges
included added weight of the metal and added energy required to
separate the hydrogen as the hydride of the metal in order to
provide it as the fuel source. What is needed in the art is a
process of making synthetic fuels from light hydrocarbons that
eliminates the problems of hydrogen storage experienced in the
prior art.
Another problem that exists in the prior art is the creation of
H.sub.2 and soot by use of a plasmatron. Although H.sub.2 and soot
may be created such as taught by Bromberg et al. in U.S. Pat. No.
5,409,784, non-ultrafine soot particle sizes are irregular, and
tend to form agglomerations that are of a size above the 2,000 nm
range. Although plasmatrons may produce H.sub.2 and soot, the
extremely chaotic nature of the production of H.sub.2 and soot
likely cause the irregular soot particle sizes.
SUMMARY AND OBJECTS OF THE INVENTION
The present invention relates to the formation of diatomic hydrogen
and elemental carbon from feed stocks of light hydrocarbons such as
natural gas and/or methane.
The inventive process operates by injecting light hydrocarbons such
as natural gas and other optional reactants into the inlet end of a
reaction chamber and rapidly heating the reactants to produce a hot
hydrogen and acetylene product stream which flows toward the outlet
end of the reaction chamber. The reaction chamber may have a
predetermined length that is sufficient to effect heating of the
reactant stream to a selected equilibrium temperature and a
preferred equilibrium composition of primarily diatomic hydrogen
and unsaturated hydrocarbons such as acetylene.
Upon reaching the selected equilibrium temperature, the desired end
product is available to be formed from the product stream as a
thermodynamically stable or unstable reaction product at a location
adjacent to the outlet end of the reactor chamber. The product
stream may be passed through a restrictive convergent-divergent
nozzle arranged coaxially within the remaining end of the reactor
chamber to rapidly cool the gaseous stream by converting kinetic
energy to thermal energy as a result of substantially adiabatic and
isentropic expansion as it flows axially through the nozzle to
minimize back reactions. A gradual expansion is not sought after as
further decomposition of the unsaturated hydrocarbon is desired.
The product stream thereby reheats to decompose the acetylene into
hydrogen and elemental carbon. Thereby, the desired end product
within the flowing gaseous stream is retained. Subsequently, the
product stream is cooled and slowed down in velocity. Solid
material, namely the elemental carbon, is recovered in any suitable
device such as a cyclone.
Preferably the rapid heating step is accomplished by introducing a
stream of plasma arc gas to a plasma torch at the inlet end of the
reaction chamber to produce a plasma within the reaction chamber
which extends toward its outlet end.
An alternate method of this invention uses a virtual
convergent-divergent nozzle. This is accomplished by directing one
or more streams of particles, droplets, liquid, or gas into the
main flow stream of the reactor chamber such that the main reactant
flow stream is forced to flow as though a real convergent-divergent
nozzle were present. This phenomena occurs because the reduced
axial momentum of the directing flow effectively impedes the flow
of the main stream, thereby forcing the majority of the main stream
to flow around the impeding stream, similar to the flow through the
restriction of a conventional converging-diverging nozzle. A
similar cooling effect is achieved with the virtual nozzle.
Although a rapid expansion is preferred in order to form elemental
carbon and diatomic hydrogen by thermal decomposition. The
directing or impeding stream(s) can play other roles than merely
providing the virtual nozzle effect. In addition to keeping the
main flow stream away from the wall, they can interact with the
main stream further downstream in various ways to provide, for
example, enhanced heat transfer, mixing, chemical reaction, etc.
The virtual nozzle effect can also be utilized in combination with
a conventional, gradual expansion converging-diverging nozzle to
achieve optimal performance of a rapid expansion. To obtain the
desired expansion and to cool the desired end products of elemental
carbon and diatomic hydrogen, it is preferable to adjust the
velocity of the reactants, the quantity of the reactants, the
number and position of the supply inlets, and diameter of the
reactor chamber.
The present invention converts a predominantly natural gas or other
light hydrocarbon stream to diatomic hydrogen and elemental carbon
with minor amounts of impurities. With either the inventive
convergent-divergent nozzle or the virtual convergent-divergent
nozzle, the present invention is particularly well suited for the
production of ultrafine solid particles comprising elemental
carbon. The rapid conversion of intermediate products to end
products allows for the formation of the ultrafine carbon
particles. Ultrafine carbon particles formed by the present
invention are in a size range from about 10 nm to about 100 nm.
Where the ultrafine solid particles are in the preferred size
range, the present invention is particularly well suited for the
storage of the diatomic hydrogen upon the ultrafine elemental
carbon.
Some benefits of the present invention include energy efficiency
and economically versatile scalability to a variety of production
rates from as low as a few thousand cubic feet per hour or lower,
to millions of cubic feet per hour or higher. The present invention
also has the benefit of sequestering a significant fraction of
carbon from the raw feed stock and diverting it from discharge to
the environment as a greenhouse gas. The present invention is also
useful for the production of hydrogen and carbon in remote areas
that only require a suitable site and an available source of
natural gas such as methane. Further any electricity needed for the
inventive process may be derived from the natural gas itself.
The present invention also uses hydrogen as a plasma gas in lieu of
argon or as a major component compared to argon. Thereby, argon
supply, and argon separation from the product gases is not required
or significantly reduced in importance.
The present invention also relates to an on-board plasma quench
reformer system for hydrocarbon fuel such as a natural gas fuel. In
the on-board plasma quench reformer, either liquid natural gas or
compressed natural gas is vaporized and converted into hydrogen and
a selection of carbon compounds including carbon dioxide, carbon
monoxide, and elemental carbon. The hydrogen is then supplied to
the internal combustion engine as a reformed fuel'source. The
carbon is not combusted, and greenhouse gases are not produced.
It is therefore an object of the present invention to provide a
method of converting one or more hydrocarbon reactants in a gaseous
stream to an end product that has the form of a gas or ultrafine
solid particle by heating the reactant stream in an axially reactor
to a preferred dissociation equilibrium of elemental carbon and
diatomic hydrogen. It is an object of one embodiment of the present
invention to accomplish the formation of diatomic hydrogen
elemental carbon by quenching the heated reactant stream and
rapidly reheating it in order to decompose any unsaturated
hydrocarbons. It is another object of one embodiment of the present
invention to accomplish decomposition of unsaturated hydrocarbons
through a free expansion or through a rapid expansion of the
equilibrium gas.
It is an object of one embodiment of the present invention to
provide a system for converting a hydrocarbon stream to diatomic
hydrogen and elemental carbon.
It is another object of one embodiment of the present invention to
rapidly expand the reactant gases by the injection of other gases
into the expansion zone to accomplish a virtual free expansion or a
virtual rapid nozzle expansion. It is another object of one
embodiment of the present invention to provide a waste heat
economizer to preheat the reactant gases for the purpose of
increased thermodynamic efficiency. It is another object of one
embodiment of the present invention to separate unsaturated
hydrocarbons from the end product stream by gas absorption and
other separations such acetylene separation from acetone. It is
another object of one embodiment of the present invention to
provide a hydrogen fuel cell system that receives the diatomic
hydrogen that has been converted from the light hydrocarbon feed
stream. It is the object of one embodiment of the present invention
to provide an on-board system that uses a hydrocarbon fuel such as
liquid natural gas or compressed natural gas to convert the light
hydrocarbon into elemental carbon and diatomic hydrogen and to
supply the diatomic hydrogen to a fuel cell or an internal
combustion engine, whereby the elemental carbon is not discharged
to the environment as a greenhouse gas.
It is also an object of one embodiment of the present invention to
provide a hydrogen storage system that combines the hydrogen and
ultrafine carbon solids produced by the inventive method.
These and other objects and features of the present invention will
become more fully apparent from the following description and
appended claims, or may be learned by the practice of the invention
as set forth hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the manner in which the above-recited and other
advantages and objects of the invention are obtained, a more
particular description of the invention briefly described above
will be rendered by reference to a specific embodiment thereof
which is illustrated in the appended drawings. Understanding that
these drawings depict only typical embodiments of the invention and
are not therefore to be considered to be limiting of its scope, the
invention will be described and explained with additional
specificity and detail through the use of the accompanying drawings
in which:
FIG. 1 is a schematic and cross-sectional view of a reactor
system;
FIG. 2 is an enlarged cross-sectional view of the reactor chamber
and converging-diverging nozzle;
FIG. 3 is a plot of temperatures, pressures, specific volumes and
nozzle throat areas as a function of gas velocity in the reactor
apparatus;
FIG. 4 is a graph plotting equilibrium concentrations in a methane
system as a function of temperature;
FIG. 5 is a cross-sectional view of a reactor chamber and expansion
chamber system wherein a gradual expansion is illustrated
thermodynamically;
FIG. 6 is a cross-sectional view of a reactor chamber and expansion
chamber system wherein a free expansion is illustrated
thermodynamically;
FIG. 7 is a cross-sectional view of a reactor chamber and expansion
chamber system wherein a free expansion is assisted by injection of
gas into the product stream to emulate an expansion nozzle;
FIG. 8 is an overview of one embodiment of the invention wherein
feed stream gas is preheated with product stream gas;
FIG. 9 is an overview of one embodiment of the invention wherein
product stream gas is further treated to separate acetylene from
hydrogen;
FIG. 10 is an overview of one embodiment of the invention wherein
product stream gas is integrated with a hydrogen fuel cell;
FIG. 11 is an overview of one embodiment of the invention wherein
product stream gas provides a supply for an on-board system;
FIG. 12 is an overview of one embodiment of the invention wherein
the elemental carbon is combined with a carbon dioxide source to
form valuable carbon monoxide.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present reactor and method are directed toward high temperature
reactions that decompose light hydrocarbons such as methane into
thermodynamically stable compounds of diatomic hydrogen and
unsaturated hydrocarbons such as acetylene, with minor impurities.
Further, the present invention accomplishes decomposition of the
unsaturated hydrocarbons to form additional diatomic hydrogen and
elemental carbon. Thereby, a liquefiable, predominantly diatomic
hydrogen fuel is prepared that is substantially devoid of any
greenhouse gas precursor chemical species.
The inventive process requires rapid cooling to freeze the diatomic
hydrogen and unsaturated hydrocarbon reaction products to prevent
back reactions or decompositions to undesirable products. The
process uses substantially constant enthalpy expansion of gases in
a device such as converging-diverging nozzle or an expansion
chamber for rapid quenching. The expansion may result in cooling
rates exceeding 10.sup.10 K/s, thus preserving reaction products
that are in equilibrium only at high temperatures. The process also
uses a free expansion of the gases after a manner that allows for
the reaction products of unsaturated hydrocarbons to further
decompose to form additional diatomic hydrogen and elemental
carbon. The process also may use the injection of gases into the
expanding gas to act as a virtual diverging nozzle or as a virtual
free expansion.
The fast quench reactor and method of operation described in this
disclosure take advantage of the temperatures in the range from
about 500 to about 20,000.degree. C. available in a high
temperature heating means such as a thermal plasma to produce
materials that are thermodynamically stable at these high
temperatures. These materials include light hydrocarbons such as
methane.
A converging-diverging (DeLaval) nozzle located downstream from the
plasma and reactant addition inlet(s) produces a rapid drop in
kinetic temperature in a flowing gas stream. This effectively
freezes or stops all chemical reactions. Rapid expansion also
permits efficient collection of desired reaction products as the
gases are rapidly cooled without achieving an equilibrium
condition. Resulting end reaction products which have been produced
in the plasma at high temperature but are thermodynamically
unstable or unavailable at lower temperatures are further
decomposed into additional diatomic hydrogen and elemental carbon
due to the rapid expansion. The end products may then be collected
due to resulting phase changes (gas to solid) or stabilization by
cooling to a lower equilibrium state.
The fast quench reactor and method of this invention shall be
described and illustrated forthwith in terms of a rapid heating
means comprising a plasma torch and a stream of plasma arc gas.
However, it will be recognized that the rapid heating means can
also include other rapid heating means such as lasers, and flames
produced by oxidation of a suitable fuel, e.g. an oxygen/hydrogen
flame.
A schematic diagram of a quenching apparatus is shown in FIG. 1. An
enclosed axial reactor chamber 20 includes an inlet 9 at one end
and an outlet at its remaining end or nozzle 22.
A plasma torch 21 is positioned adjacent to the reactor chamber 20.
Torch 21 is used to thermally decompose an incoming gaseous stream
within a resulting plasma 29 as the gaseous stream is delivered
through the inlet of the reactor chamber 20.
A plasma can be a luminous gas which is at least partially (about
1% to about 100%) ionized. A plasma is made up of gas atoms, gas
ions, and electrons. In the bulk phase a plasma is electrically
neutral. A thermal plasma can be created by passing a gas through
an electric arc. The electric arc will rapidly heat the gas by
resistive and radiative heating to very high temperatures within
microseconds of passing through the arc. The thermal plasma is
typically luminous at temperatures above 9,000 K.
A plasma can be produced with any gas in this manner. This gives
excellent control over chemical reactions in the plasma as the gas
might be neutral (argon, helium, neon), reductive (hydrogen,
methane, ammonia, carbon monoxide) or oxidative (oxygen, nitrogen,
carbon dioxide). The details of plasma generating torches are well
known and need not be further detailed within this disclosure to
make the present invention understandable to those skilled in the
art.
An incoming stream of plasma gas is denoted by arrow 31. The plasma
gas can also be a reactant or it can be inert. Preferably, the
plasma gas is hydrogen as it is a product of the inventive process
and thereby, argon separation or separation of another non-hydrogen
plasma gas from the product stream is eliminated.
A gaseous stream of one or more reactants (arrow 30) is normally
injected separately into the plasma 29, which is directed toward
the downstream outlet of reactor chamber 20. The gaseous stream
moving axially through reactor chamber 20 includes the reactants
injected into the plasma arc or within a carrier gas. A carrier gas
is also preferably hydrogen, if a carrier gas is used.
Reactant materials are usually injected downstream of the location
where the arc attaches to the annular anode of the plasma generator
or torch. Materials which can be injected into the arc region
include natural gas, such as is used in the Huels process for the
production of ethylene and acetylene from natural gas.
Gases and liquids are the preferred forms of injected reactants.
Solids may be injected, but usually vaporize too slowly for
chemical reactions to occur in the rapidly flowing plasma gas
before the gas cools. If solids are used as reactants, they will
usually be heated to a gaseous or liquid state before injection
into the plasma.
A convergent-divergent nozzle 22 is coaxially positioned within the
outlet of the reactor chamber 20. The converging or upstream
section of the nozzle restricts gas passage and controls the
residence time of the hot gaseous stream within the reactor chamber
20, allowing its contents to reach thermodynamic equilibrium. The
contraction that occurs in the cross sectional size of the gaseous
stream as it passes through the converging portions of nozzle 22
change the motion of the gas molecules from random directions,
including rotational and vibrational motions, to straight line
motion parallel to the reactor chamber axis. The dimensions of the
reactor chamber 20 and the incoming gaseous flow rates are selected
to achieve sonic velocity within the restricted nozzle throat.
Additionally, the length of reactor chamber 20 is configured to
achieve the reaction products that will facilitate the desired end
products of diatomic hydrogen and elemental carbon.
As the confined stream of gas enters the throat and diverging or
downstream portions of nozzle 22, it is subjected to an ultra fast
decrease in pressure as a result of a rapid increase in volume
along the conical walls of the nozzle exit. The resulting pressure
change rapidly lowers the temperature of the gaseous stream to a
new equilibrium condition.
An additional reactant, such as hydrogen at ambient temperatures,
can be tangentially injected into the diverging section of nozzle
22 (arrow 32) to complete the reactions or prevent back reactions
as the gases are cooled. Supply inlets for the additional reactant
gas are shown in FIG. 1 at 23.
Numerals 24 and 25 designate a coolant inlet and outlet for the
double-walled structure of the reactor chamber 20. Coolant flow is
indicated by arrows 33 and 34. The walls of nozzle 22 and a coaxial
cool down chamber 26 downstream from it may also be physically
cooled to minimize reactions along their inner wall surfaces.
Reaction product particles are collectable within a cyclone
separator shown generally at 27. A downstream liquid trap 28, such
as a liquid nitrogen trap, can be used to condense and collect
reactor products within the gaseous stream prior to the gaseous
stream entering a vacuum pump 29.
FIG. 2 further illustrates details of the converging-diverging
nozzle structure 22. The same reference numerals are used in FIG. 2
as in FIG. 1. By proper selection of nozzle dimensions, reactor
chamber 20 can be operated at atmospheric pressure or in a
pressurized condition, while cool down-chamber 26 downstream from
nozzle 22 is maintained at a vacuum pressure by operation of pump
29. The sudden pressure change is that occurs as the gaseous stream
traverses nozzle 22 brings the gaseous stream to a lower
equilibrium condition instantly and prevents unwanted back
reactions that would occur under more drawn out cooling
conditions.
Typical residence times for materials within the free flowing
plasma are on the order of milliseconds. To maximize mixing with
the plasma gas the reactants (liquid or gas) are injected under
pressure (from about 10 to about 100 atmospheres) through a small
orifice at feed stream inlet 14 to achieve sufficient velocity to
penetrate and mix with the plasma. It is preferable to use gaseous
or vaporized reactants whenever practical, since this eliminates
need for a phase change within the plasma and improves the kinetics
of the reactor. In addition, the injected stream of reactants is
injected about normal (about a 90.degree. angle) to the flow of the
plasma gases. In some cases positive or negative deviations from
this 90.degree. angle by as much as 30.degree. may be
preferred.
The high temperature of plasma 29 within reactor chamber 20 rapidly
vaporizes the injected feed stream entering at inlet 14 and breaks
apart gaseous molecular species to their atomic constituents. A
variety of products are produced, principally diatomic hydrogen and
elemental carbon with minor amounts of acetylene, and acetylene
black among others. The products can be synthesized by injecting
reactants in liquid or gaseous form into a plasma of the
appropriate gas downstream from the anode arc attachment point and
within the torch exit or along the length of the reactor chamber.
Diatomic hydrogen and elemental carbon, are especially preferred
products made according to this invention.
Reactor chamber 20 is the location in which the preferred chemical
reactions occur to preferred reaction products of hydrogen and
acetylene with minor impurities. It begins downstream from the
plasma arc inlet 9 and terminates at an outlet end 10 of reactor
chamber 20. Outlet end 10 leads to the nozzle throat 11. It
includes the reactor areas in which reactant injection/mixing and
product formation occurs, as well as the converging section of the
quench nozzle.
Temperature requirements within reactor chamber 20 and its
dimensional geometry are specific to the temperature required to
achieve an equilibrium state with an enriched quantity of each
desired reaction product of hydrogen and acetylene. Factors that
affect equilibrium include power to the plasma torch, reactor
chamber length, hydrocarbon gas flow rate and temperature, and
others. Conditions taught herein, along with conditions taught in
U.S. patent application Ser. No. 09/076,922, in copending
provisional application entitled, Thermal Device and Method for
Production of Carbon Monoxide and Hydrogen By Thermal Dissociation
of Hydrocarbon Gases, filed on May 24, 1999, and U.S. Pat. No.
5,749,937 may assist one of ordinary skill in the art to establish
other conditions for the inventive conversion of a light
hydrocarbon stream into the preferred end product of elemental
carbon and diatomic hydrogen. The disclosure of the above-cited
disclosures are incorporated herein by reference.
There is a substantial difference in temperature gradients and
gaseous flow patterns along the length of the reactor chamber 20.
At the plasma arc inlet 9, flow is turbulent and there is a high
temperature gradient; from temperatures of about 20,000 K at the
axis of the chamber to about 375 K at the chamber walls. At nozzle
throat 11, the gaseous flow is laminar, the bulk temperature is in
a range from about 500.degree. C. to about 4,000.degree. C., and
there is a very low temperature gradient across its restricted open
area. Preferably with the present invention, the temperature at
reactor outlet 10 is about 2,000.degree. C.
Since reactor chamber 20 is an area of intense heat and chemical
activity it is necessary to construct reactor chamber 20 of
materials that are compatible with the temperature and chemical
activity to minimize chemical corrosion from the reactants, and to
minimize melting degradation and ablation from the resulting
intense plasma radiation. Reactor chamber 20 may be constructed of
water cooled stainless steel, nickel, titanium, or other suitable
materials. Reactor chamber 20 can also be constructed of ceramic
materials to withstand the vigorous chemical and thermal
environment.
The walls of reactor chamber 20 are internally heated by a
combination of radiation, convection and conduction. Cooling of the
reactor chamber walls prevents unwanted melting and/or corrosion at
their surfaces. The system used to control such cooling should
maintain the walls at as high a temperature as can be permitted by
the selected wall material, which must be inert to the reactants
within the reactor chamber at the expected wall temperatures. This
is true also with regard to the nozzle walls, which are subjected
to heat only by convection and conduction.
The dimensions of the reactor chamber 20 are chosen to minimize
recirculation of the plasma and reactant gases and to maintain
sufficient heat (enthalpy) going into the nozzle throat 11 to
prevent degradation (undesirable back or side reaction
chemistry).
The length of the reactor chamber 20 may be determined
experimentally by first using an elongated tube within which the
user can locate the target reaction threshold temperature. Reactor
chamber 20 can then be designed long enough so that reactants have
sufficient residence time at the high reaction temperature to reach
an equilibrium state and complete the formation of the desired
reaction products. Such reaction temperatures can range from a
minimum of about 1500.degree. C. to about 4000.degree. C.
The inside diameter of the reactor chamber 20 is determined by the
fluid properties of the plasma and moving gaseous stream It must be
sufficiently great to permit necessary gaseous flow, but not so
large that undesirable recirculating eddies or stagnant zones are
formed along the walls of the chamber. Such detrimental flow
patterns will cool the gases prematurely and precipitate unwanted
products, such as subchlorides or carbon. As a general rule, the
inside diameter of the reactor chamber 20 should be in the range of
100 to 150 percent of the plasma diameter at the inlet end of the
reactor chamber.
The purpose of the converging section of nozzle 22 is to compress
the hot gases rapidly into a restrictive nozzle throat 11 with a
minimum of heat loss to the walls while maintaining laminar flow
and a minimum of turbulence. This requires a high aspect ratio
change in diameter that maintains smooth transitions to a first
steep angle (>45.degree.) and then to lesser angles
(<45.degree.) leading into nozzle throat 11.
The purpose of nozzle throat 11 is to compress the gases and
achieve super sonic velocities in the flowing hot gaseous stream.
This converts the random energy content of the hot gases to
translational energy (velocity) in the axial direction of gas flow.
This effectively lowers the kinetic temperature of the gases and
almost instantaneously limits further chemical reactions. The
velocities achieved in nozzle throat 11 and in the downstream
diverging section of nozzle 22 are controlled by the pressure
differential between the reactor chamber 20 and the section
downstream of the diverging section of nozzle 22. Negative pressure
can be applied downstream or positive pressure applied upstream for
this purpose, referred to as cool down chamber 26.
The purpose of the diverging section of nozzle 22 is to accelerate
and expand gases exiting nozzle 22 from sonic to supersonic
velocities, which further lowers the kinetic temperature of the
gases. It was discovered in the present invention that a free
expansion is preferable to a gradual expansion as part of the
inventive method to further decompose the unsaturated hydrocarbons
such as acetylene, into more diatomic hydrogen and elemental
carbon.
The term acceleration in practice requires use of a diverging angle
of equal to or greater than 35 degrees to expand the gases to
achieve the effects of separation from the converging wall and
inducing turbulence. Separation of the expanding gases from the
diverging wall causes recirculation of some portion of the gases
between the wall and the 11 gas jet exiting nozzle throat 11. This
recirculation in turn results in local reheating of the expanding
gases and desirable further degradation reactions, producing higher
yields of desired end products.
Physics of the Nozzle
The fast quench phenomena observed in this reactor is achieved by
rapidly converting thermal energy in the gases to kinetic energy
via a modified adiabatic and isentropic expansion through
converging-diverging nozzle 22. In the process, the gas temperature
and pressure drop extremely fast and the gas reaches supersonic
velocity. It is preferable to first raise the temperature of the
reactants in the reactor chamber to a level at which the desired
end product is more stable than other reaction products in
equilibrium with it. This is normally a consequence of the fact
that the free energy of the desired end product will decrease at
the selected elevated temperatures in comparison to the remaining
reaction products. However, this window of opportunity is very
short-lived about (<10.sup.-3 sec or shorter) in a high
temperature reactor. To stabilize maximum conversion of the
reaction product, it is necessary to rapidly cool the emerging gas
below a selected cooling temperature to force it to a lower
equilibrium state and thereby prevent decomposition of the end
product.
One distinct advantage of the present invention relates to the
production of ultrafine elemental carbon in the particle size range
from about 1 nm to about 1,000 nm, preferably from about 2 nm to
about 500 nm, and most preferably from about 10 nm to about 100 nm.
Under ordinary conditions, the generation of a plasma from a light
hydrocarbon stream will form hydrocarbons and soot materials that
are carbon particles with a diameter larger than the ultrafine
particles by a factor of at least about two and more likely by a
factor of at least about 10. Under the present invention, although
the inventors do not want to be bound to a single theory, it is
believed that the rapid expansion of ionized species creates
ultrafine carbon particles due to the freezing effect of the
ionized species before they are able to combine with a sufficient
number of other species to form soot materials as defined.
To understand the quench phenomenon in this reactor, it is
necessary to investigate the changes in the temperature, pressure,
and velocity of the gases as a function of changes in reactor
geometry.
Reactor nozzle 22 (FIG. 2) can be divided into three sections; the
convergent reactor chamber 10, the nozzle throat 11, and the
divergent quench chamber 12. The entrance angle to the throat area,
the cross-sectional area of the throat, and the diverging angle
after the throat all exert influence on the temperature, pressure,
and velocity profiles of the plasma gas.
In converging-diverging nozzle 22, the gas is flowing from a higher
pressure P.sub.0 to a lower pressure P.sub.1. During passage of the
gas through nozzle 22, there will be a rapid transformation of
thermal energy to kinetic energy. This kinetic energy will give
rise to a high gas velocity after discharging from nozzle 22. The
gas enters the converging section at a low velocity and will emerge
at the diverging section with a higher velocity.
The velocity of the gas in the throat of the nozzle, assuming
adiabatic expansion, will achieve sonic values. When the gas
accelerates through the nozzle throat, the temperature of the gas
will, simultaneously drop rapidly. As a result of high velocity
cooling, the initial gas temperature (T.sub.0) will drop to a lower
temperature, T.sub.1, upon exiting from the nozzle. This rapid
temperature quenching through a nozzle freezes the high temperature
equilibrium products of a high temperature gas phase reaction. The
pressure and temperature drop resulting from adiabatic expansion in
a converging-diverging nozzle is described in the following
equation: ##EQU1##
P.sub.0, P.sub.1, T.sub.0 are initial and final pressures and
temperatures of the gas, respectively. .gamma. is the ratio of
C.sub.p /C.sub.v where C.sub.p and C.sub.v are the heat capacities
at constant pressure and volume, respectively. At 2500 K, .gamma.
is 1.66 for Ar, 1.30 for H.sub.2, and 1.11 for C.sub.2 H.sub.2.
This equation can be used to estimate the temperature drop across
the nozzle throat if the initial and final pressures of the gases
are known or vice versa. The mass flow rate m, is related to the
cross-sectional area (A*) of the nozzle throat, the velocity (V)
and the specific volume (.OMEGA.) of the gas at the throat. The
specific volume (.OMEGA.) is the inverse of gas density at the
cross section. ##EQU2##
After substituting T.sub.0, P.sub.0, .gamma., M (molecular weight),
and R (the gas constant) for V/.OMEGA., the equation takes the
form: ##EQU3##
This equation has been used to guide the design of the nozzle
diameters used in the reactors built to date. Despite the
assumption for constant .gamma. (which is valid for an argon
plasma), the equation has been quite accurate in predictions of
mass flow as a function of temperature, pressure, molecular weight,
and nozzle diameter compared to experimental results.
The velocity of the expanding gas in mach number (Ma) is related to
temperature (T), pressure (P), density (.rho.=.OMEGA..sup.-1), and
nozzle area (A) by the following equations: ##EQU4##
In the last equation above, A* is the cross-sectional area at the
throat of the nozzle, and A is the cross-sectional area of the
converging-diverging section. Substituting T.sub.0 /T into the
equation, it becomes: ##EQU5##
FIG. 3 is a plot of T/T.sub.0, P/P.sub.0, .OMEGA..sub.0 /.OMEGA.,
and A/A* through a nozzle throat as a function of the gas velocity
(in Ma) for .gamma.=1.3 (H.sub.2). It clearly demonstrates that
both gas temperature and pressure quench rapidly upon exiting from
the nozzle. The resulting high gas velocity lends itself to the
application of a gas-turbine to recover some of the energy as
electricity to supplement the process.
Diatomic Hydrogen and Elemental Carbon Monoxide from a Light
Hydrocarbon
One preferred method for producing diatomic hydrogen and elemental
carbon from a light hydrocarbon involves directing a light
hydrocarbon gas into a hot plasma torch operated at about 50 to 100
kWhr input power at about 300 Volts DC. A hydrogen plasma gas,
preferably 100 percent hydrogen may be used. The hydrogen plasma
gas may include hydrogen as a mixture of argon and hydrogen as the
plasma gas (95% Argon: 5% Hydrogen, by volume). Because a remote
site process is contemplated, a substantially pure hydrogen source
is preferred as the plasma gas source. Accordingly, the hydrogen
gas may be manufactured on site.
The plasma gas is used to decompose the feed gas to diatomic
hydrogen and unsaturated hydrocarbons such as acetylene.
Decomposition is followed by rapid expansion of the resulting hot
gases and heating the resulting gases to further decompose the
unsaturated hydrocarbon such as acetylene. Finally cooling with
additional gases may be done to retain the diatomic hydrogen and
elemental carbon in a substantially unaltered and stable room
temperature state.
The plasma gas may be fed to the inventive reactor at a rate in a
range from about 400 to 800 cubic feet per hour. For dimensional
analysis purposes, subject to scaling up to different capacities,
the 400 to 800 cubic feet per hour plasma gas feed rate may be
matched by a light hydrocarbon feed gas rate (of, for example
methane or natural gas) of between about 200 and about 1,200 cubic
feet per hour.
FIG. 4 is an equilibrium diagram that shows methane and its
decomposition products as a function of temperature wherein
conditions cause both diatomic hydrogen and acetylene to form. In
FIG. 4, methane begins to decompose at about 1,000.degree. C. and
higher. At about 2,000.degree. C., the methane is about completely
decomposed into diatomic hydrogen and acetylene. The system
represented at about 2,000.degree. C. illustrates one preferred
equilibrium point. In the system at about 2,800.degree. C., species
such as monatomic hydrogen and C.sub.2 H begin to be formed.
The length of the reactor is chosen, in connection with temperature
requirement, such that the preferred equilibrium state of the
system exists at the nozzle throat. It is preferable to form
chemical species of unsaturated hydrocarbons such a acetylene.
Unsaturated hydrocarbons with a higher carbon-count than C.sub.2
may be formed if the size of the axial reactor is longer. It is
preferable, however, to limit the length of the axial reactor.
Thereby, the formation of unsaturated hydrocarbons is limited to
and minor impurities acetylene.
The diameter and length (about 2.0 inch.times.about 4.0 inch) of
the reactor chamber is chosen to obtain maximum mixing of plasma
gas and reactant gas while maintaining a minimum of about
2,000.degree. C. temperature at the entrance of the nozzle throat.
The reactor chamber and converging/diverging nozzle were
constructed from nickel 200 alloy to reduce corrosion. Standard
equations were used to calculate the dimensions of the bell-shaped
converging nozzle, nozzle throat diameter diverging angle, and
diverging nozzle exit diameter.
Reactants:
Methane
Plasma Torch:
10 kW laboratory plasma torch at about 75-100 kWhr electrical
energy -30 Volts, 400 Amps
Cathode: thoriated tungsten in water cooled copper
Anode: Water-Cooled Copper Cylinder 6.0 mm diameter3.times.20.0 mm
in length
Plasma Gas:
100% H.sub.2 Average total gas flow was maintained at about 200
liters/min.
Reactant Injection:
Gaseous (200.degree. C.) methane at the point where the plasma
plume exits the plasma torch. The hot methane injection tubes,
reactor chamber and converging/diverging or converging/free
expansion nozzle section were constructed from nickel 200 alloy to
minimize corrosion.
Injection Rate:
Gaseous methane was injected in a range from about 400-600
liters/min.
This resulted in a gas velocity in a range from about 3,000 ft/s to
about 5,000 ft/s.
Reactor chamber:
Water-cooled Nickel 200 cylinder 6.0 mm.times.20.0 mm
Converging Nozzle:
Bell shaped with 0.375 inner diameter radii
Nozzle throat:
2.0 mm.times.1.0 mm in length, determined from standard
equations,
Diverging Nozzle:
Conical shaped with greater than about 35.degree. included angle or
a square expansion shape to allow free expansion.
Cool down section:
Water-cooled stainless steel, 0.88 inch inner diameter
Cyclone collectors:
Water-cooled stainless steel, 12.0 mm inlet and outlet diameter,
50.0 mm inside diameter body, designed to maintain high entrance
and exit velocity
Off-Gas Cleanup:
After product collection the process gas was passed through a
liquid nitrogen cold trap and HEPA filter to remove impurities
before the gas entered the mechanical vacuum pump.
Vacuum System:
A mechanical vacuum pump was used to maintain pressure downstream
from the nozzle throat at 5.0 to 10.0 Torr (mm Hg)
FIGS. 1 and 2 pertain to an apparatus tested for converting
methane. In the described preferred embodiment, the preferred
product is diatomic hydrogen and elemental carbon and the reactant
is methane. However, the illustrated apparatus is suitable for use
with other compositions and compounds where plasma processing of
the compound requires ultra fast quenching to prevent back
reactions.
The plasma torch 21 located at the reactor chamber inlet thermally
decomposes an incoming gaseous stream comprised of a light
hydrocarbon such as natural gas or methane as the resulting gaseous
stream moves axially through reactor chamber 20 in conjunction with
a plasma gas. The resulting hot gaseous stream is then directed
through the coaxial convergent nozzle 22. The location downstream
of the convergent portion 10 of the nozzle 22 controls the
residence time of the hot gaseous stream within reactor chamber 20,
thereby allowing its contents to reach thermodynamic equilibrium.
It also streamlines the flow of hot gases, converting their motion
from random movement to straight-line movement along the central
nozzle axis. The divergent portion 12 of nozzle 22 subjects the
stream to an ultra fast decrease in pressure. Quenching streams of
gas, normally at ambient temperature, may be introduced into the
hot gaseous stream through inlets 23 as it passes through the
nozzle. This rapidly cools the contents of the hot gaseous stream
at a rate that condenses the carbon monoxide and inhibits formation
of unwanted products.
The plasma reduction is based on a quasi equilibrium-temperature
quench sequence in which the initiation of nucleation is controlled
by passage of a heated gaseous stream through a
converging-diverging nozzle geometry.
Conditions necessary for complete dissociation of a light
hydrocarbon such as methane or natural gas in a hydrocarbon stream
into diatomic hydrogen and elemental carbon can be estimated using
free energy minimization techniques which assume thermodynamic
equilibrium.
FIG. 4 illustrates that as methane is heated, decomposition thereof
is almost complete at or near about 1,000.degree. C. Where it is
desirable to minimize the occurrence of the H and C.sub.2 H
species, the reactor length is configured to freeze the system once
it reaches about 2,0000.degree. C. At this temperature, the
presence of H.sub.2 and acetylene are significant.
In a rapidly cooling plasma system one can think of the gas in
equilibrium at temperature T.sub.0 and pressure P.sub.0 being
suddenly quenched in temperature and pressure. At this temperature
the, reactants should be well equilibrated. As the plasma cools
this characteristic rate increases until at a particular T and P,
the cooling rate becomes greater than the equilibrium rate and the
composition of the plasma is frozen. On further temperature
decrease, the vapor pressure of one component subsystem becomes
greater than the saturation vapor pressure and nucleation occurs.
When this subsystem is charged, condensation is enhanced for that
species.
Experimental conditions for selective formation of diatomic
hydrogen and unsaturated hydrocarbon plasma depend on specific
values of rate coefficients and upon the initial temperature and
pressure T.sub.0 and P.sub.0 at which the plasma is frozen. Where
the unsaturated hydrocarbon is acetylene, the diatomic hydrogen and
acetylene product is also dependent on the cooling rate of the
plasma and upon the geometry of the reactor. Of course not all
reaction pathways become frozen at the same temperature during
quench.
The converging-diverging nozzle configuration used in supersonic
flow applications offers possibilities to control both the
temperature quench rate and the concentration at which the plasma
becomes frozen during the expansion. The converging-diverging
DeLaval nozzle and the associated Prandtl-Meyer expansion process
are discussed in standard texts on compressible fluid flow. In such
expansion nozzles the hot plasma gas undergoes an approximate
isentropic expansion and the energy in the gas (its enthalpy) is
converted to unidirectional velocity in the diverging nozzle. When
the exit pressure is sufficiently low, it is possible to reach up
speeds. Non-adiabatic expansion processes which are attained in
practice aid in the resultant temperature search.
A number of experiments with methane gas injected into a hydrogen
plasma are provided. The dimensions and geometry of the reactors
are varied. Provisions are made for gas quenching at the throat
exit.
Methane conversion to hydrogen and elemental carbon in an inventive
high temperature reactor and method follows the theoretical
chemical reaction: 2CH.sub.4.fwdarw.C.sub.2 H.sub.2 +H.sub.2. After
the initial decomposition, the reaction products are reheated:
C.sub.2 H.sub.2 +H.sub.2.fwdarw.2H.sub.2 +2C. In principle, under
careful kinetic studies on the pyrolysis methane it has been shown
that it is possible to obtain high yields of acetylene where the
main by-product is hydrogen, instead of tars and acetylene black.
Such studies also showed that pyrolysis in the presence of hydrogen
suppressed carbon formation in the reactor chamber.
In practice, a range of other hydrocarbons, specifically the light
olefins and solid carbon, may be formed as byproducts including
acetylene if the reaction condition under certain reactor and
process conditions.
Experiments using the fast quench system of this disclosure
revealed that the methane decomposition to acetylene is kinetics
controlled rather than equilibrium controlled. These results point
to the advantage of high quench rates which provide opportunities
to preserve high temperature equilibrium products.
EXAMPLES
The inventive method for thermally converting one or more
hydrocarbon reactants in a thermodynamically stable high
temperature gaseous stream to at least one reaction product,
comprises the following steps. The reactant stream is introduced at
one axial end of a reactor chamber containing an ionized gas. The
reactant stream is heated in the reactor chamber to form a diatomic
hydrogen and acetylene reaction product stream. The reactor chamber
has a predetermined length sufficient to effect heating of the
methane reactant stream to a temperature at which diatomic hydrogen
and acetylene are available as a major portion of a reaction
product stream, at a location adjacent the outlet end of the
reactor chamber. The reaction product stream is expanded through
the outlet end of the reactor chamber to cool the gaseous stream by
converting thermal energy to kinetic energy as the reaction product
expands. During the expansion through either a nozzle or a
free-expansion orifice, the reaction product stream reheats to
decompose the acetylene into additional hydrogen and elemental
carbon. As decomposed, the reaction product stream has been
converted into the end product stream. The end product is then
cooled to room temperature. The ionized gas is preferably hydrogen.
The end product includes diatomic hydrogen and ultrafine carbon
particles.
The quench reaction zone geometry was optimized by conducting two
dimensional modeling of the fluid dynamics of such a system.
Modeling results determined that reaction zone diameter should be
at least 150% of the plasma torch anode exit diameter. Other
diameters are 200%, 500%, and 1,000%. This allows for recirculation
of reaction gases in the reaction zone which contributes to further
decomposition of the unsaturated hydrocarbons into elemental carbon
and diatomic hydrogen.
Gas temperatures were measured experimentally along the reactor
chamber and were also modeled using a two dimensional fluid
dynamics model to determine the optimum length of the reaction zone
before the converging section. A reaction zone length was chosen
from this data for a given plasma input power level, plasma gas
flow, and reactant input rate which would result in gas
temperatures at the entrance to the nozzle throat to be equal to or
preferably greater than the required equilibrium temperature of the
desired end product.
A high aspect ratio converging section was designed such that the
radius of the convex and concave surfaces leading into the nozzle
throat were approximately equal to the diameter of the nozzle
throat. This converging geometry allows achieving the highest
possible velocity at the entrance to the nozzle throat while
limiting heat loss to the walls of the converging section or
separation of the gas flow from the converging surface.
The optimum area (diameter) of the nozzle throat was calculated
from equations available in texts pertaining to nozzle design. The
nozzle throat was designed so that with the temperature, gas
composition, mass flow, and pressure of the gas entering the nozzle
known (or estimated) sonic or near sonic gas velocities are
achieved in the nozzle throat. To achieve maximum cooling
(temperature drop) the nozzle throat should be as short as
possible. This is demonstrated by two equations for two-dimensional
nozzle flow, with R* and h* designating the radius of curvature and
throat height respectively: ##EQU6##
where T.sub.0 and a.sub.0 are the gas temperature and speed of
sound respectively in the reaction zone. In the second equation
above, all constants for a given gas are collected in C. As an
illustrative example of how to arrive at the constant C, for air,
gamma is equal to about 7/5 or about 1.4. C, it therefore equal to
about 0.278). Examination of these equations shows that greater
cooling rates occur for smaller nozzle diameters and shorter nozzle
lengths with generally smaller R* and h*.
The divergence angle and area at the exit of the diverging nozzle
were determined from standard texts on fluid dynamics and aerospace
rocket motor design. In addition, two dimensional models of fluid
flow under expected experimental conditions were also used to
optimize the divergence angle and exit area of the nozzle. A
preferred divergence angle was greater than 25.degree. and
preferably in the range from about 35.degree. to about 90.degree.
for optimum expansion and acceleration of the reaction product
stream. The maximum exit area (diameter) of the diverging nozzle
was again determined by calculation from equations available in
standard texts on fluid flow and rocket engine design.
The maximum allowable nozzle exit area depends on the mass flow
through the nozzle and pressure difference between the reaction
zone and the downstream cooling section. Choice of a large
expansion angle or a large exit area results in the gas flow
"peeling off" or separating from the wall, which results in the
preferred conditions of turbulence, gas recirculation, gas
reheating, and side further decomposition reaction to obtain the
desired end products of diatomic hydrogen and elemental carbon.
The purpose of the cool down section of the plasma fast quench
reactor device is to reduce the gas velocity while removing heat
energy (which results from the decrease in velocity) in the gas at
a rate sufficient to supply the cyclone separators effectively to
achieve a solid-gas separation between the elemental carbon and the
hydrogen.
Cool down of the reaction product gases has been accomplished by
the length of water cooled tube having the same internal diameter
as the internal exit diameter of the diverging or free expansion
section of the nozzle. With other applications of this device, it
may be more desirable to supplement gas cooling by use of other
types of heat exchangers.
Plasma quench processes for production of ultrafine materials
require product collection capability downstream of the quench
nozzle, preferably downstream of the cool down section. Cyclonic
collectors of standard dimensions described in the literature are
used for gas and mass flows several time smaller than called for in
the literature. This accommodates sonic or near sonic gas
velocities through the cyclones, which allows efficient removal of
ultrafine material (2 to 1,000 nm diameter powders).
In addition to mass flow and nozzle diameter, the third process
parameter that determines the temperature drop across the nozzle is
the ratio of the up stream pressure (P.sub.0, in reaction zone) to
the downstream pressure (P.sub.1, cool down zone). Preferably, the
ratio P.sub.0 /P.sub.1 of about 0.01 to about 0.5 is maintained.
The experimental systems were operated with the reaction zone
pressure of approximately 700 to 800 Torr (about 1 atm.) and
downstream pressure maintained between about 10 and about 200 Torr
(0.26 to 0.01 atm.). In bench scale experiments, the low downstream
pressure was accomplished using a mechanical vacuum pump.
In the following examples, the length and diameter of the reactor
chamber are configured to maintain a temperature of about
2,000.degree. C. at the entrance of the converging nozzle. The
nozzle throat diameter is configured to achieve sonic velocity of
the reaction product gas in the throat. Downstream from the nozzle
throat, gas rapidly but gradually expands, geometrically speaking.
The rapidly expanding gas acts to decompose any hydrocarbon gases
such as acetylene, ethylene, and other low molecular weight
hydrocarbons and others, that may have formed in the reactor
chamber.
Elemental carbon and other solids such as acetylene black is
separated from the end product stream by use of cyclones and the
like or filters and the like or water and/or oil spray devices and
the like.
The plasma gun is operated in a range from about 50 to about 100
kWhr input power at a potential in a range from about 100 to about
500 volts. Hydrogen plasma gas, as the preferred plasma gas, flows
at a feed rate from about 300 to about 1,000 cubic foot per hour.
Methane is supplied to the reactor chamber at a rate from about 100
to about 1,500 cubic foot per hour.
In the present invention, heating is preferably accomplished by
introducing a stream of plasma arc gas to a plasma torch at the one
axial end of the reactor chamber to produce a plasma within the
reactor chamber which extends toward its remaining axial end.
Cooling the reaction product is preferably accomplished by use of a
restrictive convergent-divergent nozzle or a free expansion.
FIG. 5 is a cross-sectional view of a reactor chamber and expansion
chamber system 126 where a rapid expansion is illustrated
thermodynamically. In comparison to previous work done by the
inventors, a gradual expansion of reaction gases was realized to be
an impediment to the formation of elemental carbon by the
substantially complete decomposition of the unsaturated
hydrocarbons. A gradual expansion of the reaction gases allowed the
acetylene and/or other unsaturated hydrocarbons that were being
formed in the reactor chamber, to resist further decomposition. In
the previous technology developed for the production of such
products as elemental titanium, a gradual expansion to prevent
reheating and back reactions was important due to the highly
reactive nature of the halides from which the elemental titanium
had just been dissociated.
In contrast thereto, the present invention, preferably decomposes
the unsaturated hydrocarbons to the point of obtaining elemental
carbon. As such the remaining compounds are substantially all
diatomic hydrogen. The hydrogen may then be liquefied and used in
processes such as an internal combustion engine or a hydrogen fuel
cell. The combustion of hydrogen produces no greenhouse gases as
illustrated: H.sub.2 +1/2 O.sub.2.fwdarw.H.sub.2 O.
In FIG. 5, the angle alpha is preferably in a range greater than
about 25 degrees, preferably greater than about 35 degrees to
expand the gases to achieve the effects of separation from the
diverging wall and inducing turbulence. In the previously disclosed
technology, it was preferable to have a nozzle angle less than
about 35 degrees in order to accomplish a gradual expansion. In the
present invention, separation of the expanding gases from the
diverging wall causes recirculation of some portion of the gases
between the wall and the gas jet exiting nozzle throat 11. This
recirculation in turn results in local reheating of the expanding
gases and desirable further degradation reactions, producing higher
yields of the desired end products of elemental carbon and diatomic
hydrogen.
FIG. 5 also illustrates constant enthalpy of the gas as it moves
through nozzle throat 11. As kinetic temperature is significantly
reduced in nozzle throat 11, the reaction gases accelerate to
hypersonic speeds. As such, temperature is dropped. The present
invention comprises a method of thermally converting one or more
hydrocarbon reactants in a gaseous stream to an end product in the
form of a gas or ultrafine solid particles. First, a reactant
stream is introduced at one axial end of a reactor at a preferred
space time velocity. The preferred space time velocity is dependent
upon the actual dimensions of the reactor and is calculated to
achieve a preferred equilibrium state at nozzle throat 11.
A first heating of the reactant stream is carried out as the
reactant stream flows axially toward outlet end 10 of the reactor.
The reactor has a predetermined length sufficient to effect heating
of the reactant stream to a selected temperature at which at least
diatomic hydrogen and at least one unsaturated hydrocarbon product
is available at or near equilibrium as an intermediate or reaction
product stream at a location adjacent outlet end 10 of the
reactor.
The intermediate or reaction product stream is expanded through a
nozzle arranged at the outlet end of the reactor to rapidly cool
the intermediate or reaction product stream by converting thermal
energy to kinetic energy as a result of substantially adiabatic and
isentropic expansion as the intermediate product stream flows
axially through the nozzle. Thereby, the desired intermediate
product stream is retained without undesirable side and back
reactions.
A second heating is carried out on the intermediate product stream
sufficient to decompose the intermediate product stream to
principally a diatomic hydrogen and elemental carbon end product.
Finally, the at least one end product and remaining intermediate
product stream exiting from the nozzle are cooled to room
temperature.
The reactant stream is preferably selected from methane, natural
gas, and hydrocarbons. The hydrocarbons are preferably light
hydrocarbons such as propane, butane, pentane, and hexane.
Typically, the light hydrocarbons may include such impurities as
nitrogen and sulfur as elemental constituents or as compounded
constituents.
Preferably, as illustrated in FIG. 4, the first heating of the
reactant stream comprises heating the reactant stream to about
2,000.degree. C. as the reactant stream approaches nozzle throat
11. As such the first of heating the reactant stream is
accomplished in a space time velocity that substantially prevents
the formation of unsaturated hydrocarbons with more than two
carbons therein. This preferred space time velocity allows for a
more complete decomposition of the acetylene or other unsaturated
C.sub.2 hydrocarbons during the second heating.
The inventive method may include expanding the intermediate product
stream through a convergent-divergent nozzle that has an angle,
alpha, that allows for reheating of the product gases sufficient to
decompose the unsaturated C.sub.2 hydrocarbons during the second
heating as illustrated in FIG. 5.
FIG. 6 illustrates alternative reheating of the product gases by
allowing the product gases to experience a substantially free
expansion into an expansion chamber 226. Reheating occurs during
the free expansion sufficient to decompose the unsaturated C.sub.2
hydrocarbons during the second heating to principally elemental
carbon and diatomic hydrogen. The free expansion experienced by the
intermediate product stream experiences significant turbulence that
allows the intermediate product stream to further decompose.
Decomposition may be done by kinetic and thermal means.
FIG. 7 also illustrates the method of thermally converting one or
more hydrocarbon reactants in a gaseous stream to an end product in
the form of a gas or ultrafine solid particles. This alternative
includes expanding the intermediate product stream through a free
expansion 226 or a rapid or gradual divergent nozzle, followed by
blending the intermediate product stream with a mixing gas, either
by injecting dilution gas at an angle to axial flow through
injectors 23 (FIG. 2), or by injecting directly into the axial flow
through an injector 123, or a combination of both. Blending causes
the intermediate product stream to become diluted and individual
species from the intermediate product stream to become separated by
the mixing gas.
As illustrated in FIGS. 5-7 expanding the intermediate product
stream through a nozzle arrangement comprises supplying the
intermediate product stream through a convergent nozzle at a
velocity at which kinetic temperature drops by an absolute
temperature factor of greater than about two. Preferably, the
temperature drops from about 2,000.degree. C. to about 500.degree.
C. The second heating may raise the temperature up to about
800.degree. C. in order to accomplish decomposition of the
intermediate product stream to at least a diatomic hydrogen and
elemental carbon end product.
Expanding the intermediate product stream through a convergent
nozzle with either a divergent nozzle or a free expansion may also
be done at a velocity at which kinetic temperature drops by an
absolute temperature factor of greater than about three.
Additionally, expanding the intermediate product stream through a
nozzle or a free expansion nozzle may also be done at a velocity at
which kinetic temperature drops by an absolute temperature factor
of greater than about four.
In order to decompose the unsaturated hydrocarbons in the
intermediate product stream, the second heating of the intermediate
product stream is therefore preferably done by heating the expanded
intermediate product stream by an absolute temperature factor of
greater than about 1.25. Preferably, the second heating of the
intermediate product stream is therefore preferably done by heating
the expanded intermediate product stream by an absolute temperature
factor of greater than about 1.4.
FIG. 8 illustrates a flow scheme in which the diatomic hydrogen,
prior to separation from the elemental carbon, is used in an
economizer heat exchanger 136 to preheat feed stream gases such as
methane, natural gases, or other light hydrocarbons. Alternatively,
the economizer heat exchanger may be located at a flow point after
the elemental carbon and other solids has been removed. The mass
and energy flow rates depicted in FIG. 8 are for non-limiting
illustrative purposes as examples of one scale of production.
FIG. 9 is an overview of one embodiment of the invention wherein
product stream gas is further treated to separate acetylene from
hydrogen. In some applications, it may be preferable to allow some
unsaturated hydrocarbons to pass through the expansion without
total decomposition to elemental carbon and diatomic hydrogen.
Where such an end product mix may be produced, separation of
acetylene from hydrogen may be accomplished by acetone absorption
of the acetylene. FIG. 9 illustrates the acetone absorption of
acetylene in an absorber 138, followed by separation of acetylene
from acetone in a separator 140. Other absorbents of the acetylene
may be used, however, and other separation schemes may be employed
such as membrane separation and the like. The mass and energy flow
rates depicted in FIG. 9 are for non-limiting illustrative purposes
as examples of one production sale. Absorber 138 and separator 140
are one example of a means for separating unsaturated hydrocarbons
from the end product stream.
FIG. 10 is an overview of one embodiment of the invention wherein
product stream gas is integrated with a hydrogen fuel cell 142.
This embodiment of the present invention is preferred where
efficient generation of electrical power is needed, such as at a
remote site, and where the waste heat that is generated in the fuel
cell may be used as an economizer heat exchanger for the inventive
process. FIG. 10 illustrates a first waste heat economizer 136 that
assists in cooling the exit gases from the rapid expansion chamber
by preheating the feed stream. After the separation of elemental
carbon from the end product stream (not pictured), the diatomic
hydrogen is used as a secondary feed stream to a hydrogen/light
hydrocarbon fuel cell. Waste heat from the hydrogen/light
hydrocarbon fuel cell may also be used to preheat the primary feed
stream in a second waste heat economizer 144.
Although fuel cell 142 may be a hydrogen fuel cell or a
hydrogen/light hydrocarbon fuel cell due to the presence of some
amounts of undecomposed light hydrocarbons in the end product
stream, it is also an embodiment of the present invention to
provide light hydrocarbons to fuel cell 142, either alone or in
concert with the diatomic hydrogen end product stream. The mass and
energy flow rates depicted in FIG. 10 are for
non-limiting-illustrative purposes as examples of one scale of
production.
FIG. 11 illustrates another embodiment of the present invention.
The inventive conversion of light hydrocarbons to elemental carbon
and diatomic hydrogen is used in an on-board system. The inventive
conversion of light hydrocarbons to elemental carbon and diatomic
hydrogen is used in the on-board system.
FIG. 11 is an overview of the on-board system for an internal
combustion engine 146 that may be powered either by a hydrocarbon
fuel such as natural gas or by the diatomic hydrogen that is
generated on board. The on-board system may be operated by feeding
a hydrocarbon fuel such as natural gas in a feed stream 156 to
reactor chamber 20. Conversion of the hydrocarbon fuel is carried
out according to the present invention, whereby elemental carbon
and diatomic hydrogen exit and pass through first waste heat
economizer 136 and into cyclone 127. Cyclone 127 is one example of
a means for recovering the carbon product and the diatomic hydrogen
product.
Elemental carbon exits at stream 150, and diatomic hydrogen exits
in stream 148. Internal combustion engine 146 receives hydrogen
stream 148 and optionally may also receive a hydrocarbon stream
158. After combustion, effluent gases pass to second waste heat
economizer 144. Optionally, a hydrocarbon feed stream 154 may
originate at second waste heat economizer 144 in order to preheat
the natural gas feed stream. The preheated natural gas feed stream
exits second waste heat economizer 144 and enters first waste heat
economizer 136, after which it passes there through and enters
reactor chamber 20 as heated feed stream 160. Optionally, a
hydrocarbon feed stream 152 may begin at first waste heat
economizer 136 and be fed to reactor chamber 20 as preheated feed
stream 162.
It can be seen that the light hydrocarbons may be supplied to the
on-board system, either as a direct fuel for internal combustion
engine 146 or as a fuel to be reformed into hydrogen as streams
156, 160, and 162. Any combination of the above feed streams may
also be used according to a specific application. Elemental carbon
stream 150 is allowed to exit the on-board system and may be
collected and used as a precursor or feed material for other
processes.
A preferred embodiment of the invention depicted in FIG. 11
includes the exclusive feed of hydrocarbons through feed stream
154. This embodiment comprises hydrocarbon feed stream 154 passing
through second waste heat economizer 144, next passing through
first waste heat economizer 136 and supplying reactor chamber 20 as
preheated feed stream 160. In this preferred embodiment preheated
feed stream 160 has attained a temperature that is higher than
preheated feed stream 162 or feed stream 156 if they had been used.
Thereby, the amount of energy that is needed to heat preheated feed
stream 160, Q.sub.IN is reduced. The on-board system depicted in
FIG. 11 uses natural gas or some other light hydrocarbon for its
fuel. Because of the conversion of the light hydrocarbon to
elemental carbon and diatomic hydrogen, and because elemental
carbon stream 150 is not discharged to the environment as a
greenhouse gas, the on-board system provides energy Q.sub.OUT with
a substantially clean-burning fuel that was converted from a
potential source of greenhouse gases.
FIG. 11 demonstrates that either liquid natural gas (LNG) or
compressed natural gas (CNG) may be used to fuel internal
combustion engine 146.
FIG. 12 illustrates one use of elemental carbon stream 150 as
illustrated in FIG. 11. Where it is desirable to utilize the
elemental carbon; whether as part of an on-board system or in a
separate operation, the elemental carbon may be contacted with
conventional combustion exhaust gases to react with the carbon
dioxide to form carbon monoxide. Carbon monoxide is a valuable
intermediate for many industries. The prior technology for
production of carbon monoxide involved steam and/or catalytic
reforming of natural gas to form the products of carbon monoxide
and diatomic hydrogen.
The present invention utilizes the abundance of carbon dioxide,
that is produced by the large number of operations around the
globe, to generate the valuable intermediate of carbon monoxide. An
inventive system is provided in FIG. 12 that takes advantage of
combination of the inventive conversion of a light hydrocarbon into
elemental carbon and diatomic hydrogen, with the conversion of the
elemental carbon with carbon dioxide into the valuable carbon
monoxide.
FIG. 12 illustrates a gas turbine 168 that burns a light
hydrocarbon such as methane to turn a generator 170 for the
generation of power sufficient to operate the inventive plasma
quench to convert a light hydrocarbon such as methane. Combustion
products exit gas turbine 168 as principally carbon dioxide and
water according to the reaction: 2O.sub.2 +CH.sub.4.fwdarw.CO.sub.2
+2H.sub.2 O. Where preferable, a condenser 164 may be provided to
remove water from the combustion gas effluent from gas turbine
168.
The inventive plasma quench operates from generator 170 that is
used to strike a plasma in order to convert methane or other light
hydrocarbons within reactor chamber into elemental carbon and
diatomic hydrogen. A waste heat exchanger 236 may be provided,
either before or after at least one cyclone 227 for separating the
elemental carbon from the diatomic hydrogen.
The elemental carbon that exits cyclone 227 may then be combined
with the carbon dioxide effluent from gas turbine 168 in a carbon
monoxide-producing reactor 166. Reactor 166 may be a fluidized bed
reactor where the carbon dioxide is combined with the elemental
carbon under reactive conditions according to the reaction:
C+CO.sub.2.fwdarw.2CO.
Other reactor schemes may be used for the combination of elemental
carbon with carbon dioxide to form carbon monoxide. The carbon
stream may be heated to a highly reactive solid species or it may
be heated above about 4,200 .degree. C. to a gaseous species.
The carbon monoxide produced from reactor 166, or any other reactor
scheme known in the art, is a valuable feed stock for several
processes such as the alcohols industry and others.
The present invention may be embodied in other specific forms
without departing. from its spirit or essential characteristics.
The described embodiments are to be considered in all respects only
as illustrated and not restrictive. The scope of the invention is,
therefore, indicated by the appended claims rather than by the
foregoing description. All changes which come within the meaning
and range of equivalency of the claims are to be embraced within
their scope.
* * * * *